Cooling water systems are subject to formation of scale deposits. Scaling can occur when the concentration of a dissolved substance in a cooling water becomes greater than its solubility in the water. It can especially be a problem with a substance that has an inverse solubility curve; that is, a material whose solubility decreases as the temperature increases. Since water temperatures at or near heat-transfer surfaces are greater than temperatures in the bulk of the system, the solubility of such materials is less in these regions. Consequently, they tend to precipitate and form scales that reduce heat-transfer efficiency.
One principal scale-forming material encountered in cooling water systems is calcium carbonate formed by the decomposition of calcium bicarbonate. This compound not only has an inverse solubility curve, but its solubility is much lower in most typical cooling waters than almost all other potential scale-formers that might be present in these waters. Of course, calcium carbonate is soluble in acidic solutions, and as the pH of a cooling water is lowered, scale generally becomes less of a problem. However, most cooling waters are kept on the alkaline side to reduce corrosion, and thus calcium carbonate scaling remains as a potential problem. Calcium sulfate, calcium phosphate, barium sulfate, and ferric hydroxide can also cause scale. Thus, to be a broadly useful composition, a scale control product must be capable of controlling different scale types.
It is well known that the operation of commercial and industrial cooling systems is adversely affected by a number of different factors. Of these adverse factors, corrosion of metallic parts coming into contact with the water is probably one of the most serious. If not controlled, corrosion causes the rapid deterioration of the metallic materials of construction used in cooling towers and associated equipment such as pumps, pipelines and valves, causing major losses in overall efficiency of the cooling systems. While control of bleedoff, pH, and other operating variables is helpful in reducing corrosion, chemical treatment of the water is generally the most effective and economical means of minimizing this problem, particularly where conservation of water by means of recycling is necessary or desired.
Waterside problems encountered in boilers and steam systems include the formation of scale and other deposits, corrosion, and foam. Scale and other deposits on heat-transfer surfaces can cause loss of the thermal efficiency of the boiler and can make the temperatures of the boiler metal increase. Under scaling conditions, temperatures may go high enough to lead to failure of the metal due to overheating. Corrosion in boilers and steam systems also causes failure of boiler metal and damage to steam and condensate lines.
The principal source of deposits in boilers is dissolved mineral matter in the boiler feedwater. The term "scale" is generally used for deposits that adhere to boiler surfaces exposed to the water, while nonadherent deposits are called "sludge" or "mud." Scale causes more difficulty because the sludge can be purged from the system with the blowdown or can be easily washed out, but scale can normally only be removed by mechanical or chemical cleaning of the boiler.
In natural, untreated water, the main sources of scale and sludge are calcium carbonate, calcium sulfate, magnesium hydroxide, and silica. The most common type of scale in boilers is probably calcium carbonate, but the most troublesome is usually calcium sulfate. The latter causes more difficulties because its solubility decreases more rapidly with increasing temperatures than does that of other substances, and the scale it forms is hard, dense, and difficult to remove. On the other hand, calcium carbonate tends to form sludge more than scale, and the calcium carbonate scales that do form are generally softer and easier to remove. Magnesium hydroxide precipitates are not very adherent and tend to form sludges rather than scales.
It is often desirable in today's technology to prevent precipitation of alkaline earth salts or of iron salts from water or aqueous solutions. For this purpose inorganic and organic sequestering agents have previously been proposed and utilized. For instance, the organic compounds nitrilo triacetic acid or ethylenediamine tetraacetic acid have been used. Likewise polymeric phosphates have also been used as sequestering agents. The latter have the advantage that they can prevent precipitation even if applied in less than a stoichiometric amount. The disadvantages of the polymeric phosphates, however, are that they lose effectiveness at elevated temperatures and that they readily hydrolyze, particularly in the acidic pH range. For reasons related to sewage disposal, additional problems may develop in the use of phosphates. It has already been proposed to use organic phosphonic acids, such as non-substituted aminotrimethylene phosphonic acid, for this purpose, but it has been found that corrosion problems occur therefrom.
Aminoalkylenephosphonic acids and their metal and ammonium salts are well known compounds and recognized metal complexing agents. The quaternization of such compounds in an aqueous solution has not heretofore been achieved. Under normal reaction conditions, there is apparent protonation (or zwitter ion formation) of the free electrons on the nitrogen atoms in the molecule. Conditions which are sufficiently alkaline to remove said proton tend to destroy the alkylating agent faster than it can react in the desired fashion.